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Ice creams take on the Sydney heatwave

An Icy Pole and a chocolate gelato are subjected to Friday's 40+ degree heat. How many minutes do you think they took to melt?

The heatwave projections, released on Friday during what was expected to be Sydney's hottest three-day spell in November in almost eight decades, cover the period to 2030 and then out to 2070.

The shift towards hotter weather is already evident, with south-eastern NSW experiencing about 18 more heatwave days a year compared with the start of the 20th century, the Office of Environment and Heritage says.

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For most other parts of the state, the increase was about four-11 days.

The research, based on 12 climate models as part of the NSW and ACT Climate Modelling Project (NARCliM), estimates that most of the state will experience 1-1½ more heatwave events a year by 2030.

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The number of heatwave days - defined to be excessive heat compared with historical records and the preceding 30 days - will increase by as much as 10 days a year by 2030 in the state's north, with smaller increases near the coast. (See chart below showing the rising percentage of heatwave days each year.)

The heat starts building early in the morning across Sydney on Friday. Photo: Peter Rae

"This provides a clear indication that, out to 2030, we can expect the heatwaves to happen more often, and for them to be longer," Matthew Riley, director of OEH's Climate and Atmospheric Science, said.

Given the inertia of the climate, the 2030 increase in heatwave frequency and duration are basically locked in, no matter what happens to greenhouse gas emissions.

A couple pours water on their dogs to escape the Sydney heat. Photo: Daniel Munoz

"That's likely to occur out to 2030, regardless of the emissions scenario," Mr Riley said.

Extending the current emissions trajectory, the models found that the number of extra heatwave days a year would soar by 26-33 days in the state's north by 2070. The south-west and south-east would fare better, with about seven additional heatwave days annually. (See chart below).

"[The models show] even more heatwaves out to 2070, that last longer still, and are becoming hotter," Mr Riley said.

In general, coastal regions will fare better than inland ones because of the proximity to sea breezes. While the phenomenon is evident even on Friday - with the CBD recently at 30 degrees while Penrith to the west was more than 40 degrees - the research indicates some interesting microclimates may develop around Sydney.

As the city sprawls to the north-west and south-west to accommodate 1.5 million more people in the two decades to 2031, western Sydney and the Hawkesbury will experience five-10 more hot days by 2030, a related OEH report said.

The replacement of vegetation such as forest and grasslands with concrete can raise morning summer surface temperatures by an average of 12.8 degrees - underscoring the importance for city planners to retain or introduce more green spaces in their designs.

"The more green cover you have in urban environments, the cooler the conditions," Mr Riley said, noting that funds are available to aid councils to reduce the so-called urban heat island.

One quirk in the research is that building higher density housing in inner-city suburbs may help mitigate the heat effects.

"We think that may be due to increased shading effects and the way the [taller buildings] can influence how the sea breeze forms and how it penetrates inland," Mr Riley said.

The effects appear strongest in the afternoon (see chart below), although Mr Riley stressed that any temperature reduction would most likely be much smaller than the background shift towards warmer overall temperatures and an increase in heatwave days.